In the conventional techniques regarding diodes, the light-emitting diode (LED) can be used as a pixel 11 of a monitor 10 (as shown in FIG. 1). Since the LED has excellent optoelectronic features of low power consumption, low heat generation, long operation life, good impact resistance, small size, fast responding speed, and stable emission wavelength, etc., the LED monitor has become one of the important directions of development in display devices currently. However, the light emitted by LED is of single-wavelength, hence the display device using a LED as a pixel can be merely used as a monochrome monitor.
With the progress of the age, the development in display devices is emphasized on making full-color monitors. To meet the challenge from the change of the time and the demands of users, the full-color LED monitor has become the main stream of the market gradually.
Please refer to FIG. 2. Each pixel 20 is composed of three LEDs in the conventional full-color LED monitor 25. That is, LED 21, LED 22, and LED 23 as shown in FIG. 2 can emit red light, green light, and blue light respectively. Therefore, the pixel 20 composed of three basic colors can emit full-color light. Moreover, please refer to FIG. 3. Full-color LED monitor 35 is the improvement of full-color LED monitors 25 shown in FIG. 2. Each pixel 30 is composed of four LEDs in the full-color LED monitor 35. That is, LED 31, LED 32, LED 33, and LED 34 as shown in FIG. 3 can emit red light, green light, blue light, and white light respectively. Therefore, once the LED 34 is electrically conducted, the pixel 30 can emit the white light. The description of the full-color LED monitor 35 can be referred to U.S. Pat. No. 5,998,925.
The structure of conventional LED adopted in the aforementioned traditional full-color LED monitor is shown in FIG. 4. The LED shown in FIG. 4 comprises a coating resin 401, a LED chip 402, a conductive wire 403, a molding material 404, a lead frame 405 and an inner lead 406, wherein the lead frame 405 comprises a base 405a and a lead 405b. The description of the aforementioned LED structure can be referred to U.S. Pat. No. 5,998,925. Hereinafter, the LED structure shown in FIG. 4 will be described in details.
Such as shown in FIG. 4, the coating resin 401 is filled in the base 405a to cover the LED chip 402, so as to prevent the LED chip 402 from contacting oxygen or moisture, thereby protecting the LED chip 402. The coating resin 401 is generally made of transparent material, such as epoxy resin, urea resin, or glass, etc. However, the thermal expansion coefficient and heat conductivity of the coating resin 401 are apparently different from those of the LED chip 402, so that the heat generated from the imperfect electro-optical conversion is easy to be accumulated on the interface between the coating resin 401 and the LED chip 402, while the optoelectronic element is in operation. Moreover, in the manufacturing process, it is quite important about how to use proper temperature and process for the coating resin 401 to be stably coated on or filled in the area surrounding the LED chip 402, and meanwhile, to assure that no extra chemical reaction between two different materials (the coating resin 401 and the LED chip 402) will occur. However, with the current technology, it usually needs to perform a baking step on the coating resin 401 at 150° C. for about 40 minutes, so as to cure the coating resin 401. Hence, for fitting to the current process, the coating resin 401 of high purity has to be selected as the material used for coating or filling (since some elements are easy to be diffused into semiconductor material to change the original properties of the semiconductor material).
The aforementioned structure also causes another bad influence. As the coating resin 401 is a poor heat conductor, heat is accumulated on the interface between two different materials (the coating resin 401 and the LED chip 402). Due to the difference in the thermal expansion coefficients between the coating resin 401 and the LED chip 402, while the element is in operation, heat accumulated therein causes additional stress exerted on the LED chip 402, wherein the stress is exactly proportional to the interface temperature (which is caused by the accumulated heat). While LED elements are developed towards the applications of high brightness and high power, the aforementioned problem will become more and more serious. Even on the current common applications, since the coating resin 401 and the LED chip 402 are different in material properties, the operation stability and life of the optoelectronic element are affected directly or indirectly.
Further, the LED chip 402 is a semiconductor element having a PN junction. Hence, when a positive voltage is applied to two electrodes of the LED chip 402, the light of specific wavelength will be emitted from the PN junction of the LED chip 402. In the aforementioned structure, the light emitted by the LED chip 402 towards the base 405a cannot be emitted again to the external, and thus the light emission intensity and efficiency of the entire LED device are affected. However, under the current structure, these shortcomings are inevitable.
Such as shown in FIG. 4, the coating resin 401 is used to fill in the base 405a to cover the LED chip 402, and the coating resin 401 may comprise fluorescent matter, such as phosphor. Besides, the coating resin 401 can be transparent material, such as epoxy resin, urine resin, or glass, etc. Moreover, the fluorescent matter contained in the coating resin 401 can change the light emission wavelength by the way of energy conversion, and the porosity and coating thickness of the fluorescent matter also affect the color of the colored light emitted after the wavelengths respectively generated by the LED and the fluorescent matter are mixed. However, on one hand, due to the oxidization reaction and the deterioration scheme of the coating resin 401, and on the other hand, due to the temperature influence and the UV light irradiation, the deterioration of the coating resin 401 and phosphor is thus accelerated. When the coating resin 401 is deteriorated and cured because of heat, or is damaged by the UV light in sunshine, the coating resin 401 has the phenomenon of curing and deteriorating. Once the coating resin 401 starts deteriorating, the LED chip 402 covered thereby will be affected and damaged. Especially for the element of which the waveband of light emitted is below that of blue light (wherein the wavelength of emitted light is smaller than 480 nm), because the LED chip 402 thereof has the attribute of spontaneous light-emission, and additionally, the light traveling path thereof is concentrated within a specific angle, resulting in high light emission intensity, consequently, the damage to the coating resin 401 is more sever. With the occurrence of these situations, the light-emitting device has the chance to be functionally retarded.
In the process for manufacturing the conventional LED, the LED chip 402 has to first be fixed on the base 405a. Thereafter, the conductive wire 403 is formed between the LED chip 402 and the inner lead 406 in a manner of wire bond. Then, the coating resin 401 is filled in the base 405a to cover the LED chip 402 and part of the conductive wire 403. However, errors may occur in the process of fixing the LED chip 402, and the conductive wire 403 may not be able to be formed accurately on the bonding pad of the LED chip 402 while being formed on the LED chip 402, thus causing the LED chip 402 to be electrically nonconductive, resulting in manufacturing a defective LED.
To sum up, for the current development of full-color display devices, it is a problem about how to provide a full-color LED display device that can increase the light-emitting efficiency and prevent the light-emitting element (such as the aforementioned LED chip 402) from being damaged by the coating resin.